Historically, the harvesting and curing of tobacco has presented the greatest bottleneck to efficient, economical production. Tobacco leaves were hand harvested over a 5 to 8 week period and the leaves strung on sticks which were placed on tiers in curing barns 16 to 24 feet high. Labor requirements for harvest through curing were in the range of 180 to 220 man-hours per acre. Mechanical harvesting and bulk curing systems introduced during the 1960's reduced these requirements to about 50 man-hours per acre.
The advent of mechanical harvesting introduced a new problem, however, from a material handling viewpoint. A machine is capable of harvesting up to 12 to 15 tons of material per day. This meant that hand racking of tobacco at the barn now became the bottleneck to further gains in efficiency. Recognizing this fact in my previous applications, Ser. No. 288,028, now abandoned, Ser. No. 425,848, now U.S. Pat. No. 3,932,946, and Ser. No. 388,590, now U.S. Pat. No. 3,899,836 a modular tobacco handling and curing system is described, wherein large containers are positioned onto a special plenum for curing and air is forced upwardly or downwardly through the containers.
My research has indicated certain limitations to the current approaches, particularly in regard to achieving maximum economy in the handling and curing system. With large containers, tobacco is inherently packed to a higher density than in the normal racks and barn capacity is increased. With the small racks, the average density of tobacco in the curing space is 6 to 8 pounds/ft.sup.3 ; whereas, with the large containers, the average density may be 14 to 18 pounds/ft.sup.3. For timely curing and to achieve high quality, it is necessary that static pressures in the inlet plenum be increased to move air at the required rate through the more dense tobacco. Furthermore, since the drying front moves progressively from the inlet to outlet, increasing this distance through which the air must be forced has the effect of increasing power requirements for the fan and increasing the length of the drying period, factors which are contrary to the goal of improving curing efficiency. Another very important factor is that the tobacco within the container near the exit side is subjected to near saturated air conditions throughout the yellowing phase and for some time into the drying phase until a considerable portion of water in the tobacco has been removed from leaves near the inlet side. While this may be satisfactory for many leaf conditions, a number of conditions have been noted in which leaf deterioration can rapidly occur. Under certain field conditions, depending upon weather, soil moisture and temperature, soil pathogens, (particularly Erwinia) can develop rapidly and be splashed onto lower leaves of the plant. During curing, at high humidities, infected leaves can develop soft rot and rapidly deteriorate both midrib and laminar portions of the leaf. During the past two years, several cases of soft rot have been observed to occur in large curing containers where tobacco was infected by leaf rotting pathogens, with losses up to 50%. It appears evident, that there are strict limitations to the practical distances through which the air passes through tobacco in large containers.